Pinning the hand on the lower back
The spanker takes with his or her non-spanking hand a firm grip of the spankee's wrist or hand and pins the spankee's hand at the small of the back clear out of the way above the buttocks. The spanking can then be continued with no further interference.
The method is typically used in the over-the-knee position and its variants, the over-one-knee position, the straddling position, the frontal over-the-lap position, and the tucked-under-the-arm position. Usually the spankee will only manage to bring one of their hands back, because the other one is blocked by the spanker's body. In the case that both hands fling back, the usual method is to take both wrists in one hand.
This way the spankee experiences renewed helplessness, which is why in consensual spanking some spankees will often willfully "try to block" the spanking so that their "naughty hand" can be put away and secured, and the spanking continued.
On the other hand, some spankers take pride in never having to do this as they have taught their charges to keep their hands in front no matter what and never, never try to reach back, on the threat of horrible extra punishment. An example of such discipline in spanking fiction is found in the series Collegegirl by Lurking Dragon (1999).